The Italian air force fielded, in early days of European war, a considerable force of decent, but not great fighters.
What fighters should Italians have in production use, for the specified time frame (roughly a contemporary of Spitfire), while using the easily available 'bits pieces'? Ie. existing Italian engines armament, then-modern airfoils, radios, layout etc. You can contemplate single and twin engined fighters.

1938. Austrian unification with Germany.
.....Success this time because Italy diplomatically supports the plebiscite.
.....Hitler expressed his undying gratitude to Mussolini for Italian diplomatic support.

Point of Departure. 1938.
"Chancellor Hitler, Italy requires more then gratitude. We need a license to produce the latest model DB601 engine and engineering assistance to build a factory similiar in size to Genshagen (220 engines per month). This engine will power our new Macchi C.202 fighter aircraft. We would like to begin factory construction NLT Summer 1938. Perhaps you can cut through German red tape to make this possible. Then Italy will consider your desire to unite Sudetenland with Germany."

Modern inline engines were the key, of course. Italy had airframes like the MC200 and G50 crying out for better engines. On the other hand, even if the Italians had produced a world beater in 1940 their airforce would still have been dysfunctional due to the nepotism and incompetence in the command structure.

Engine power I agree, though italy and germany working together was always a huge pie in the sky until after italy had entered the war. The italians were keen to cash in on German success only after it had been demonstrated, until that point, and the point of their own failures fully exposed, they were more interested in the concept of "parrallel war", proving that italy was still the senior power in the fascist Axis alliance.

So, in my book, angling for DB engined Macchis (or anything else really) is as big a pipe dream as thinking they might be provided with Merlin engines.

Without German (or allied) help, Italian engine development is stalled, so what can they reasonably do under their own steam (though they did produce great racing engines from a very early stage.....why these were not deveoped into combat types is a bit of a mystery) .

The Macchi C200, was designed by Mario Castoldi, who had gained a lot of experience in the early '30s with aerodynamically clean schneider cup racers. It is surprising then to see the MC 200 come up as such a poor aerodynamic design, with lumps and bumps, open cockpit and such light armament to boot. If i were in charge, with no restrictions (other than to use Italian resources) I would adapt the CR 42 to a ground attack role, abandon the fiat design and concentrate on improving the aerodynamica and armament of the Macchi.

If the Macchi had been cleaned up and given an armament of 4 x HMG, or 2 x HMG and 2 x LMG, one might expect performance to improve to about 330 mph, and firepower to be about 2/3 of that of the RAF types. Not a lot would have been needed to achieve that sort of outcome

The Macchi C200, was designed by Mario Castoldi, who had gained a lot of experience in the early '30s with aerodynamically clean schneider cup racers. It is surprising then to see the MC 200 come up as such a poor aerodynamic design, with lumps and bumps, open cockpit and such light armament to boot.

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Supermarine/Mitchell had Schneider trophy winning experience too but look at the performance of the Type 224.

It seems some interim developmental step might be meeded between a pure racer and the adoption of much of the lessons learned into a really effective fighter. It's a long,long way from the Supermarine S.6B to K 5054,much further than much "popular" history would have us believe.

A certain young Canadian aerodynamicist who had been working in Germany, Beverley Shenstone,did not join Supermarine until June 1931

Perhaps if the Alfa Romeo 135 double pegasus had been successfully developed the Italians would have access to a 1,600 bhp radial on 87 octane and up 2,000 bhp if they sourced 100 octane. Mate that to an MC200 style airframe, or Fiat G50 style (the Finns liked it) and push the .5" Beda-SAFATs (or better Scottis) into wing batteries of 3 per wing.

All Italian (well Italian developed from Bristol) and it first ran in 1938. They already had a San Giorgo reflector gunsight and adequate .5" guns.

The Italians don't need to struck deals with Germany to get themselves a decent V-12, they have their own. Issota Fraschini Asso IX could be described as the '90% of DB-601A', the Asso L.121 being almost as good as the DB. The IX was in production in 1937, the L.121 was powering a competitor for the CR.42. The hub cannon for the DB was a tricky item for Germans until mid/late 1940 anyway.
So I'd went for the Asso-engined plane (the MC.202 look-alike), 4 HMGs (either in wing roots, synchronized; or, in wings, like Wildcat was carrying), 300-350 rpg, ~400 L of fuel (or roughly 100-120 USG) between pilot and engine. The twin engined plane with same engines could be a classic twin (ie. layout like Fw-187, DH Hornet etc), carrying 250-300 US gals and up to 8 HMGs.

How much the Italians know about DB-601 in time the new fighter(s) should be in the prototype stage; how much they know about German intentions factories for the engine?
They know what the Asso XI can deliver in the time the 1st drawings are being made, and they certainly have that engine powering the prototype, some time in 1937? They also know what to expect from the Asso L.121 prior the Germans themselves received the 1st DB-601.

Once the Italians find out about the DB-601, lets say they make demands and succeed. The time for the Italians to introduce the DB-601 (building of the new factory, manpower training, engine testing, mating the airframe with engine, testing of the upgraded aircraft) would be what, 1.5-2 years, if the Italo-German connection works flawlessly? (they emulate Packard RR combo?! the Packard beaing already well versed in engine manufacturing) Just when this time-line ends - the new fighter being too late for 1940, let alone 1939.

The Issota-Fraschini management can dump the Delta, so the engineers can try and upgrade the Asso in the meantime.

hell of you are going to get a license to produce engines...why not go one step further and produce the whole plane....109s. other countries bought them from germany before the war. when germany went to war with the Yugoslvians it was one of the few times 109s fought against 109s.

The Macchi C200, was designed by Mario Castoldi, who had gained a lot of experience in the early '30s with aerodynamically clean schneider cup racers. It is surprising then to see the MC 200 come up as such a poor aerodynamic design, with lumps and bumps, open cockpit and such light armament to boot.

If the Macchi had been cleaned up and given an armament of 4 x HMG, or 2 x HMG and 2 x LMG, one might expect performance to improve to about 330 mph, and firepower to be about 2/3 of that of the RAF types. Not a lot would have been needed to achieve that sort of outcome

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I'm not so sure the MC200 was so unaerodynamic. After all, it was almost as fast as the opposing Hurricanes, with 250 hp less. Those lumps and bumps on the engine cowling covered the cylinder heads of course, so the only way to eliminate them would be to increase the diameter of the entire cowling, which would be a bad option. And the MC200 was designed with an enclosed cockpit - Italian pilots flew without it by preference. Many of the British and Soviet pilots they flew against did likewise.
Armament was an issue of course. At the end of the day the Italians had good designers and airframes ready to go, but they lacked the engines and guns to put in their fighters and that left them playing catch up. But their biggest problem was the incompetence of their commanders and force structure. Even once they had the MC202 they lacked the logistical nous to keep such advanced aircraft flying under front line conditions. Servicibility rates were terrible. As Walter Boone said; the P-40s and Hurricanes couldn't outfly the MC 202, but they could outlast it.

I'd concentrate on the Macchi Mc.202 or the Fiat, pick one, doesn't matter which. The engine doesn't matter as much as building it and getting an air force mattered. Pick an engine and an airframe and get to work.

Too bad they didn't, but they didn't. With war loomingm they still didn't do it.

1930s Italy spent a huge amount of money conquering Ethiopia and establishing military facilities in East Africa. That's why modernization of the Italian armed forces lagged behind neighboring nations. 1930s Japan was in the same boat. Most of the military budget was required to support ongoing military operations in China with little remaining for modernization.

1930s Italy can have Ethiopia or they can have modern aircraft and tanks. They cannot afford both.